Planetary Persistence

The formation of Earth-sized planets all across the cosmos is more robust than previously thought.

The tally of exoplanets—planets outside the solar system—continues to climb, with more than 900 cataloged as of this writing. All but a handful of those alien worlds have turned up orbiting stars that, like our Sun, are isolated, rather than stars in the crowded stellar groupings known as open clusters. Most stars originate in clusters that quickly break apart. Particularly dense starry swarms, however, can persist for billions of years thanks to the mutual gravitational attraction of their thousands of constituent stars.

Given the lack of exoplanet detections in long-lived open star clusters, astronomers have assumed those chaotic environments to be hostile to worldrearing. And that is in keeping with the core accretion model, the leading theory of how planets arise, which holds that planets slowly form, piecemeal, from aggregating grains of dust in a delicate disk around young stars. That model requires “peace and quiet,” explains Søren Meibom of the Harvard- Smithsonian Center for Astrophysics in Cambridge, Massachusetts, lead author of a new study on small-planet occurrence in open clusters. In tightly packed stellar quarters, gravitational tugging and pulling by nearby stars can disrupt planet-forming disks and fling developed planets out of orbit. Furthermore, open clusters often spawn the biggest, hottest stars. The radiation spewing from those behemoths—as well as their explosive demises as supernovae—makes planetary formation seem an even tougher proposition.

So it came as a surprise when astronomers led by Meibom discovered two exoplanets in the billion-year-old open cluster NGC6811. NASA’s Kepler space telescope spotted the planets crossing their stars, dimming out a tiny fraction of starlight. This so-called transit method of detection enables astronomers to estimate a planet’s size. The new planets, the first revealed in a cluster by this method, are also the smallest seen in a cluster: larger than Earth, but not quite Neptune-size. Prior to their discovery, only four planets, all of them giant, had been found in star clusters.

The fact that those smallish planets can form and survive in such a roughand-tumble place challenges our understanding of planet formation whilesuggesting that all across the cosmos, planets are hard to root out. “The result is telling us that the process of forming planets is more robust than we had believed it was,” said Meibom. “This is good news for the idea that planets like the Earth should be everywhere.” (Nature)